Without foregoing other possible uses of the present invention in the field of voice and/or data communication via an air interface, only one application in the mobile radio sector will be discussed in the text which follows. Products in the PC, PDA and/or mobile radio sector are known as complete terminals and as plug-in or extension units. They are subject to high price pressure, this field of application being distinguished by particularly high requirements for the signal quality with very little available space for a corresponding circuit and strict restrictions with regard to the energy demand. Accordingly, the method and device of the type initially mentioned will also find application in the very small communication devices according to the Global System for Mobile Communication GSM and the Unified Mobile Telecommunications system UMTS standard.
For increasing the usable channel capacities, system considerations for communication systems and mobile radio devices beyond the third generation of mobile radio systems go in the direction of multi-transmitter and/or multi-receiver systems. In this context, the possible transmittable data rate is increased by utilizing a further dimension, namely space, by means of multi-path propagation. These methods are known from the literature by the following names                Multiple Input Multiple Output, MIMO in brief,        Single Input Multiple Output, SIMO in brief, and        Multiple Input Single Output, MISO in brief.        
As a rule, they use a number of complete transmitting and receiving paths, these paths being accounted for jointly in the digital signal processing. Accordingly, each transmission path requires, e.g., a receiving chain with antenna, preamplifier, mixer, filter and A/D converter. All systems operating in accordance with the MIMO principle have in common that different data streams are transmitted at the same frequency and at the same time in one channel, this channel usually being designed for a SISO application with regard to the bandwidth provided for it.
The data streams are assembled or separated algorithmically at the transmitting and receiving end in the respective signal processing arrangement. There are MIMO system proposals which operate with or without feedback from a receiver to the transmitter. In the systems with feedback, the characteristics of the MIMO transmission channel are estimated at a time T1, then the channel information is transmitted via a return channel to the transmitter so that the latter can carry out a corresponding predistortion of the transmitting signal which is sent out at time T2, with the aim of maximizing the data throughput. The result of the channel estimation is a complex matrix, the rank of which corresponds to the number of transmission paths which form a MIMO transmission channel from part-transmission channels. The assumption here is that the propagation conditions do not significantly change between the times T1 and T2 so that the channel estimation is still correct. These methods are known in principle from the literature.